1. Field of the Invention
The present invention relates to an automatically self adjusting annular jet mixer useful in mixing guar and other materials to create a fracturing fluid gel at the site of a gas or oil well.
2. Description of the Related Art
Mixing of guar and other material for creating a fracturing fluid gel has been known for approximately 50 years. Fracturing fluids are used to carry or transport proppant, usually sand, into a well fracture for the purpose of creating improved production of hydrocarbons, i.e. oil or natural gas. In the past, guar gel has had quality problems which were evident by lumps of partially hydrated gel within the gel fluid. These lumps could possible plug off formation permeability and also caused reduced viscosity of the gel. The reduced viscosity was caused by not all of the gel being incorporated into the fluid and thus not being fully utilized. Many efforts, some quite elaborate, have been used to produce a quality gel, i.e. one that was free of lumps. Screens have been used to filter out lumps. Grinders and shear devices have been used to break down the lumps. Chemicals have been used to coat the dry gel powder particles to slow the hydration process and thereby prevent lumps. Guar powder has also been mixed as slurry with diesel fuel to create a concentrated suspension for later mixing into a gel. All these techniques added cost to the material, and depending on the process, added elaborate and expensive equipment. All of these solutions added to the cost of fracturing a well, thus making the produced oil and gas more expensive.
Mixing energy has been found to be an important key to mixing a lump free gel. Guar powder tends to lump if it is not fully wetted when it first encounters water. Thus, a high energy mixer that wets all guar powder particles will create a lump free gel. One of the problems with standard mixers is that the nozzle or jet from which the water exits is usually fixed in size, i.e. the nozzle is not adjustable. If the process rate is changed from the optimal flow for that nozzle, then the performance is changed. If the process rate is less than the optimal rate, then not enough energy will be created to mix the gel free of lumps. In the process rate is much higher than the optimal rate, a high pressure loss is developed in the nozzle which increases required pump horse power and further limits the maximum throughput rate. The most economical fracturing process is one in which the gel is prepared “on-the-fly” at the same time the fracturing fluid is pumped down the well. Guar does need some time to hydrate and develop the desired viscosity. Therefore, a holding tank downstream of the mixer is usually needed before the fluid is mixed with the proppant and is then pumped down the well. Since the characteristics of wells vary greatly, there is a need to mix guar gels at different rates, depending on the stage and well treatment design. The present invention provides a high energy mixer that also automatically adjusts the nozzle size to maintain a high energy nozzle jet to efficiently mix the gel at a wide range of flow rates. The adjustment means employed in the present invention requires no outside power source or control means, whether electronic, mechanical or hydraulic. The water that is used to mix the gel also creates the power that is used to adjust the mixer nozzle. A pressure reducing valve operating on the process water is used to adjust the mixer pressure setting. Once this setting has been made, no other future adjustments are necessary.